Spatio-temporal distribution and abundance of <Emphasis Type="Italic">Euphausia crystallorophias</Emphasis> in Terra Nova Bay (Ross Sea,Antarctica) during austral summer

Average abundance values for Euphausia crystallorophias in the shelf region of Terra Nova Bay were generally high. Mean number of adults and juveniles were 20 and 87 ind/m², respectively, while larval concentrations were much higher, reaching a mean of 14,764 ind/m². Euphausia crystallorophias were very patchy in their distribution during all stages of their life cycle. The distribution patterns of E. crystallorophias in TNB seem to be strictly correlated both to hydrological features such as the presence of polynyas, pack-ice zone and certain temperatures values. According to our data, E. crystallorophias spawned from early October to early January. For the two age groups (0+ and 1+) an average growth rate of 0.074 and 0.076 mm/day was found, respectively. The mean biomass of larval E. crystallorophias (from Metanauplius to Furcilia I) in January was estimated as 84 mg WW/m². This article belongs to a special topic: Five articles on Sea-ice communities in Terra Nova Bay (Ross Sea), coordinated by L. Guglielmo and V. Saggiomo, appear in this issue of Polar Biology. The studies were conducted in the frame of the National Program of Research in Antarctica (PNRA) of Italy.     

Euphausia superba dominates in the diet of Adélie penguins feeding under fast sea-ice in the shelf areas of Enderby Land in summer

Adélie penguins Pygoscelis adeliae in Enderby Land, Antarctica feed mainly on Euphausia superba during the chick rearing season in shelf areas where fast sea-ice remains: indicating that E. superba is abundant under the fast sea-ice in these areas. The shelf areas in Enderby Land, therefore, are unique since the previous studies of Adélie penguin diet in Ross Sea, Adélie Land and Prydz Bay show that E. crystallorophias is the most abundant krill species in shelf areas in general.     

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.     

The developmental rate ofEuphausia crystallorophias larvae in Ellis Fjord,Vestfold Hills,Antarctica

 Euphausia crystallorophias is the dominant zooplankton species in the neritic seas of Antarctica, where it occurs in similar abundances to those of Euphausia superba in more offshore areas. Despite its great abundance and probable ecological significance, few details are known of this species’ development, life history and ecology. This study found that E. crystallorophias spawned in Ellis Fjord from late November to early December and completed its larval development under the sea ice during the Antarctic winter. The mean time for E. crystallorophias eggs to develop to furcilia stage VI was 235.5 days, which is virtually identical to the developmental time already reported in the laboratory, but almost twice that of E. superba. This slow development rate is likely to be due either to the low water temperatures (<0^°C) in which E. crystallorophias lives, or to low levels of food being available over winter. Received: 30 August 1995/Accepted: 11 December 1995     

Horizontal and vertical distribution and demography of euphausiids in the Ross Sea and its adjacent waters in 2004/2005

The horizontal and vertical distribution and population structure of euphausiids in the Ross Sea and its adjacent waters were investigated during the summers of 2004/2005 using stratified towed samples. Nine species of euphausiids occurred in the survey area. Among them, Euphausia triacantha was dominant in biomass north of the southern boundary of the Antarctic circumpolar current (SB). Thysanoessa spp. was widely distributed north of the continental slope, while E. superba was distributed from the SB to the slope, where it showed the highest biomass. Juvenile E. superba was distributed offshore near the SB and remained at the surface, but gravid females were dominant in the slope and mainly occurred in the middle layers (400–600 m). Adult and juvenile E. crystallorophias were found at 200–300 m in the colder water of the continental shelf. In general, the peak biomass of euphausiids was found in the mid layers of the Ross Sea area. The life span and the number of spawns for major species are also discussed.     

Zooplankton communities in the Southern Weddell Sea (Antartica)

Summary Standardized abundances of 40 frequently occurring macrozooplankton taxa collected by double-oblique bongo net hauls between the surface and 200 m depth were submitted to an agglomerative hierarchical cluster analysis, to characterize surface zooplankton communities in the southern and eastern Weddell Sea. The sensitivity of the analysis concerning reduction of the number of considered taxa (especially stages of species) was tested. Dominant taxa in the entire area over the whole period were copepods Metridia gerlachei and Calanoides acutus. Calanus propinquus, Appendicularia spp. and calyptopis-1 larvae of krill, Euphausia superba were abundant at 73°S/19°W. Euphausia crystallorophias and larval fish Pleuragramma antarcticum dominated at 77°S/40°W on the southern shelf. The cluster analysis revealed marked similarities between the southern shelf community and the January community further to the northeast. E. crystallorophias, and larval Pleuragramma antarcticum are thought to be transported in surface layers of the coastal current to the southwest. Oceanic species such as Thysanoessa sp. and E. superba are probably not transported in the same way. Possible mechanisms for the maintenance of the community structures are discussed. A simplified method of characterizing communities, based on analysis of euphausiids, is presented.     

The potential impact of a krill fishery upon pelagic fish in the Prydz Bay area of Antarctica

Summary RMT hauls taken during FIBEX in the Prydz Bay region showed a relatively high incidence of fish compared with most previous reports from the Atlantic sector. Overall, fish comprised 3.8% by number and 8.1% by weight, and these figures rose to 26.8 and 46.1%, respectively in aimed hauls over the continental shelf. Even when only hauls with>50% E. superba are considered, fish accounted for almost 1% by weight of the catch overall, and more in aimed hauls or those taken over the continental shelf. Species composition differed markedly between the continental shelf and deep water zones. Over the shelf the fish catch comprised mainly Pleuragramma antarcticum and channichthyid juveniles, while over deep water it consisted mainly of myctophids and Notolepis coatsi. These data indicate that the potentially large krill fishery in this region could affect the populations of some shelf dwelling demersal species by increasing the mortality among their pelagic juvenile stages.     

Distribution,Abundance and population structure of the Antarctic krill (Euphausia superba Dana) in the Prydz Bay region,Antarctica

Summary This paper presents results of net sampling carried out on four marine science cruises, between 1981 and 1985, in the Prydz Bay region of Antarctica by the Australian Antarctic Division. Krill exhibited a patchy distribution and overall low abundance. The majority of sampling sites in January 1985 returned no post-larval krill or densities of <1 individual 1000 m^-3. The estimated mean abundance of E. superba in January 1985 was 6 individuals or 2 g (wet wt.) 1000 m^-3 integrated for the upper 200 m of the water column which represented 3.4% of the total zooplankton biomass. No more than five years-groups, including the larvae, were observed in Prydz Bay, with mean lengths of groups 1+, 2+, 3+ and 4+ being 24, 38, 46 and 53 mm (standard 1), respectively in the middle of January. A high proportion of naupliar stages observed in January 1985 indicated that spawning in Prydz Bay begins in January and examination of adult maturation showed that the spawning continues at least to March.     

Electrophoretic investigation of genetic variation in two krill species Euphausia superba and E. crystallorophias (Euphausiidae)

Summary Specimens of Euphausia superba and of E. crystallorophias from different locations were analyzed electrophoretically for protein variation. The present study extends previous genetic studies on E. superba by analyzing samples from the East Wind Drift and repeat samples from the Bransfield Strait, Elephant Island and the Weddell Sea. E. crystallorophias was collected in the Weddell Sea and around the Antarctic Peninsula in order to provide information on the breeding structure of the species in this region. For all loki taking all sampling sites together for both species except GPI in E. crystallorophias no significant deviation of phenotype distributions from random mating expectations was observed. Furthermore, the allele distributions in all polymorphic loci for both species were found to be homogeneous. Estimates of genetic distance between samples within each species are low (0.0001 to 0.0003 in E. superba and 0.0001 to 0.0002 in E. crystallorophias), and are consistent with results expected for samples from a single interbreeding population. Estimate of genetic distance between these two species was 0.9729. These results suggest that for each species specimens from all locations investigated in the Bransfield Strait and Weddell Sea belong to a single genetically homogeneous population. A possible mechanism for maintaining such homogeneity and the implications for fishery management are discussed.     

Comparisons of morphology and neritic distributions of <Emphasis Type="Italic">Euphausia crystallorophias</Emphasis> and <Emphasis Type="Italic">Euphausia superba</Emphasis> furcilia during autumn and winter west of the Antarctic Peninsula

Euphausia crystallorophias and E. superba larvae often overlap in distribution in Antarctic coastal regions. Here, we describe the morphology and ecology of E. crystallorophias furcilia stages F3–F6, with emphasis on characteristics that distinguish them from E. superba, based on samples collected west of the Antarctic Peninsula during autumn and winter 2001 and 2002. During autumn most E. crystallorophias occurred as F4s (53%) and F5s (35%), while E. superba occurred in all furcilia stages (F1–F6). During winter, F6 was the dominant stage (>67%) for both species. On average, body lengths of E. crystallorophias larval stages were significantly greater than those of E. superba. During autumn, densities of the two species were similar (range: 0.003–11.8 m^–3) at many on-shelf stations, with lower densities during winter. Where both species occurred, >58% of E. crystallorophias furcilia were collected between 50 and 100 m depth, while 82% of E. superba were shallower (25–50 m). Younger stages of E. crystallorophias occurred more frequently (54% of F3s) in water >100 m than older stages (11% of F6s). Thus, many larval E. crystallorophias were vertically segregated from E. superba, thereby reducing grazing competition between the young of these morphologically similar species.     

A comparison of Antarctic euphausiids sampled by net and from geothermally heated waters: insights into sampling bias

The Antarctic euphausiids Euphausia superba and E. crystallorophias were sampled using an 8 m² rectangular midwater trawl inside the caldera of Deception Island. Specimens of the same species were collected on the same day from the strandline and sublittoral fringe within the caldera at a bay where these euphausiids had been incapacitated by high temperature geothermally heated waters. Differences between length frequency distributions of samples obtained by both methods were investigated once net-caught specimens had been subject to the simulated effects of geothermally heated waters. Differences between maturity stage/sex ratios for E. superba samples were also assessed. No significant differences between sizes of net-caught or beach-sampled E. crystallorophias were detected, but significant differences were found between both the sizes and sex/maturity stages for E. superba samples. Possible explanations for these differences are discussed. Accepted: 15 March 1999     

Larvae of euphausiids off Queen Maud Land

Summary In a March 1981 study of the Lazare Sea, a sector of the Southern Ocean along Queen Maud Land, Euphausia superba and Thysanoessa macrura larvae were abundant in the north and northwest part of the region near the frontal zone between the Weddell Warm Countercurrent (WWC) and Weddell Gyre, 5°W–15°E. Toward the south, E. superba and E. crystallorophias larvae were numerous in the frontal zone between the WWC and the Antarctic Coastal Current. Only T. macrura larvae occurred in the central part of the sea. The age composition of larvae was most advanced in T. macrura, probably due to the drift of larvae from the northwest. Because the ice cover diminishes later in the flow from the Weddell Gyre (northernmost area of the region) than in the south, E. superba larvae were younger in that oceanic subregion than near the shore. T. macrura larvae were sparse near the coast and an onshore-offshore age difference was not pronounced. Both frontal zones correspond to reproductive areas of E. superba and T. macrura. E. crystallorophias spawns only in the southern area.     

A comparison of Antarctic petrel (Thalassoica antarctica) diets with net samples of Antarctic krill (Euphausia superba) taken from the Prydz Bay region

Samples of the stomach contents of Antarctic petrels (Thalassoica antarctica) were obtained on board ship in the Prydz Bay region of Antarctica from birds which had spontaneously regurgitated. The weight of food and the species composition of the stomach contents were measured. Antarctic krill,Euphausia superba, the sole prey item taken, were compared to krill obtained by nets in the same region as part of a large-scale krill survey. Krill from petrel stomach samples were larger in mean size than those sampled by nets. This may be attributed to selection of the larger sized krill by the petrels, it may be caused by the nets sampling different populations of krill or it may be due to net avoidance by the larger krill.     

Meso- and macrozooplankton communities in the Weddell Sea,Antarctica

Summary The present paper describes composition and abundance of meso- and macrozooplankton in the epipelagic zone of the Weddell Sea and gives a systematic review of encountered species regarding results of earlier expeditions. Material was sampled from 6 February to 10 March 1983 from RV Polarstern with a RMT 1+8 m (320 and 4500 m mesh size). In agreement with topography and water mass distribution three distinct communities were defined, clearly separated by cluster analysis: The Southern Shelf Community has lowest abundances (approx. 9000 ind./1000 m³). Euphausia crystallorophias and Metridia gerlachei are predominating. Compared with the low overall abundance the number of regularly occurring species is high (55) due to many neritic forms. Herbivores and omnivores are dominating (58% and 35%). The North-eastern Shelf Community has highest abundances (about 31 000 ind./1000 m³). It is predominated by copepodites I–III of Calanus propinquus and Calanoides acutus (61%). The faunal composition is characterized by both oceanic and neritic species (64). Fine-filter feeders are prevailing (65%). The Oceanic Community has a mean abundance of approximately 23 000 ind./1000 m³, consisting of 61 species. Dominances are not as pronounced as in the shelf communities. Apart from abundant species like Calanus propinquus, Calanoides acutus, Metridia gerlachei, Oithona spp. and Oncaea spp. many typical inhabitants of the Eastwind Drift are encountered. All feeding types have about the same importance in the Oceanic Community.     

The biology of the spiny icefish <Emphasis Type="Italic">Chaenodraco wilsoni</Emphasis> Regan, 1914

The most abundant ice fish species observed in catches off the northern tip of the Antarctic Peninsula in the last 25–30 years has been the spiny ice fish Chaenodraco wilsoni Regan 1914. C. wilsoni has been exploited on a commercial scale from the late 1970s to the end of the 1980s off Joinville–D’Urville Islands (CCAMLR Statistical Subarea 48.1) and in the Cosmonauts and Cooperation Seas and Prydz Bay in the Indian Ocean sector (CCAMLR Statistical Division 58.4.2). This paper presents new information on biological features and life history characteristics of C. wilsoni, based on research survey collections along the northern Antarctic Peninsula in 2006 and 2007 and samples taken in the commercial fishery in 1987. Length frequency compositions from the research surveys demonstrated that fish 21–34 cm long predominated in the catches. Sexual maturity is attained at 24–25 cm. Absolute fecundity and relative fecundity is low (1,000–2,500 eggs; 6–12 eggs). Oocyte diameter varied from 4.0 to 4.9 mm very close to spawning. Spawning at the tip of the Antarctic Peninsula is likely to occur in October–November. Remotely operated vehicle deployments in the northern Weddell Sea demonstrated that C. wilsoni exhibit parental nest guarding where males protect the eggs. The incubation period is likely to be 8 months long. Fish feed primarily on Antarctic krill (Euphausia superba) in the Antarctic Peninsula region and in the Cosmonauts and Cooperation Seas while fish take ice krill (Euphausia crystallorophias), Pleuragramma antarcticum and myctophids to some extent in other areas. Age determination still awaits validation. Preliminary ageing attempts suggested a maximum age of about 8–10 years.     

Zooplankton biomass and abundance of Antarctic krillEuphausia superba DANA in Indian ocean sector of the southern ocean

Zooplankton sampling was carried out during the first six Indian Scientific Expeditions to Antarctica (1981—1987) to estimate krill abundance in the Indian sector of the southern ocean (between 35° to 70° S and 10° to 52° E). This study aims to understand the distribution of biomass of zooplankton, especially the krill, using the data collected by net sampling techniques. Total zooplankton biomass for all the sampling stations ranged from 9 to 684 ml/1000m³ (x: 143.34±138.61 SD; n=150). Biomass data grouped by 5° latitude interval shows the presence of higher biomass (x: 191 ml/1000 m³;n = 31) in the Antarctic divergence region (between 60° to 65° S). EuphausiidEuphausia superba DANA formed a considerable component of biomass and together with eggs and larvae represented over 60% (n = 39) of mean total zooplankton-biomass in the coastal ice region. The values for population density and biomass of adult-size krill varied between 0 to 4320 individuals and 6 to 305 ml/1000 m³, respectively. The highest values were recorded between 62° to 69° S and between 16° to 30° E. A high density of krill larvae was encountered in the shelf region during January 1987 which was related to chlorophyll concentration. However, the values obtained during six consecutive summers showed that values of adult krill biomass at given location was highly variable and, hence, not comparable on inter-annual basis, during the sampling years.     

Diet of Adélie penguins Pygoscelis adeliae during the post-hatching period at Esperanza Bay,Antarctica, 1987/88

The diet of the Adélie penguin Pygoscelis adeliae was studied at Esperanza Bay, Antarctic Peninsula, during the post-hatching period by quantitative analysis of adult stomach contents. Euphausiids constituted on average 96% by mass, while fish contributed 4% by mass. Amphipods were present in small amounts. Antarctic krill Euphausia superba was the predominant component throughout the sampling period. In contrast, Euphausia crystallorophias occurred rarely. Differences in the size of krill taken by Adélie penguins appear to reflect either local changes in the availability of certain age classes at various times in the breed ing season, or differences in foraging areas, or are due to year-to-year differences in prey availability and abundance, possibly caused by variations in seasonal ice cover. Pleuragramma antarcticum constituted the bulk of the fish portion, particularly during the guard period.     

Stomach contents of the Antarctic Silverfish Pleuragramma antarcticum from the southern and eastern Weddell Sea (Antarctica)

Summary The stomach contents of 142 Pleuragramma antarcticum from the southernmost part of the Weddell Sea (Gould Bay: 77°19S) and from the eastern coast (72°–74°S) were analyzed. The fish were collected in February 1983 and stored frozen. Size range of the investigated specimens was from 6.4 cm SL (7.3 cm TL) to 21.9 cm SL (24.1 cm TL). The fish were caught by bottom trawl (Gould Bay) and Agassiz Trawl and pelagic RMT-m net in the upper 300 m layer at the eastern coast, respectively. Thirty-six stomachs were empty or contained sand only. The most important prey in terms of biomass were euphausiids, which occurred in 49 stomachs at a mean number of 2. Their dry weight was estimated to be 15–50 times that of the next important food items, which were copepods, gastropods and gammarids. Krill (Euphausia superba) constituted up to 14% of the euphausiids in the diet of fish caught in the eastern Weddell Sea. The rest was usually made up by Euphausia crystallorophias. This species was taken with preference even when the abundance of E. superba in accompanying plankton catches was 16 times higher. In the Gould Bay, gastropods were frequently eaten despite high numbers of copepods in the plankton, whereas in the eastern Weddell Sea, copepods were abundant both in plankton and in the stomach content. The almost complete absence of the early stages of fish in the diet of Pleuragramma antarcticum in the southern and eastern Weddell Sea is due to a pronounced vertical segregation of fish of different sizes. This distribution pattern is thought to be an adaptive mechanism to avoid intraspecific predation, as 90% of the ichthyoplankton in that area is Pleuragramma antarcticum.     

1998/1999年南极普里兹湾邻近海域浮游植物的分布特征

中国第15次南大洋考察从普里兹湾邻近海域获得25个测站的浮游植物样品,主要研究了其种类组成、分布及其与环境的关系.浮游植物有5门16科21属48种(变种和变型),浮游植物平均细胞密度为22.46×103个/dm3,其中以硅藻类占优势(84.51%).浮游植物分布以近海岸陆架区的细胞密度最高(46.03×103个/dm3),其次为陆坡(4.40×103个/dm3),深海区最低(3.34×103个/dm3).表层叶绿素a浓度为0.16～3.99 μg/dm3,普里兹湾内和湾西部四女士浅滩海域浓度在3.5 μg/dm3以上;平面分布趋势浓度从湾内向西北方向递减,深海区浓度在0.5 μg/dm3以下.浮游植物优势种为硅藻的短拟脆杆藻(Fragilariopsis curta).浮游植物垂直分布密集区位于0～50 m水层,100 m或100 m以下水层随深度的增加而细胞密度逐渐减少,200 m水层稀少或未见.其密集区位于普里兹湾近岸陆架区,而陆坡及深海区细胞密度显著减少.叶绿素a浓度的最大值同样分布在25 m或50 m层,50 m以下的浓度随深度的增加而降低,200 m层叶绿素a浓度分布范围为0.01～0.95 μg/dm3.粒径分级叶绿素a浓度以微小型浮游生物的贡献占优势(56%),微型浮游生物的贡献占24%,微微型浮游生物的贡献占20%.经回归统计分析,浮游植物细胞丰度(y)与水温(T)、盐度成正相关,与营养盐(PO4 (P)、NO-3 (N)、SiO3 (Si))成显著负相关.     

Feeding of dominant zooplankton in Prydz Bay, Antarctica, during austral spring/summer: food availability and species responses

Information on the food and predation of zooplankton species is essential for an improved understanding of zooplankton community dynamics of the Southern Ocean. Feeding of dominant zooplankton species at locations across the neritic, slope and oceanic regions of Prydz Bay, Antarctica, was investigated with incubation experiments during austral spring/summer of 2009/2010 to identify the response of dominant copepods and krill to different feeding environments. Results showed distinct spatial and temporal variations in ingestion and filtration of predominant copepods and krill. In late spring, Calanoides acutus was inactive and the ingestion rate was only 1.30 μgC/ind/day. During early summer, a diatom bloom was in progress at station IS21, showing a peak biomass of Thalassiosira spp. and Fragilaria spp. Daily ingestion rates of Euphausia crystallorophias, C. acutus, Metridia gerlachei and Ctenocalanus citer were relatively high. By contrast, copepod ate predominantly ciliates in slope and oceanic regions where microplankton biomass were lower (<20 μgC/L). During late summer, microplankton of neritic regions, mainly composed of nano-sized Pseudo-nitzschia spp. and ciliates, was less than 10 μgC/L. C. acutus incubated in neritic regions mainly ate ciliates. The total microplankton biomass was lower (<5 μgC/L) and predominated by Pseudo-nitzschia spp. in slope regions north of Fram Bank, and daily ingestion rates of incubated copepods were less than 2.5 μgC/ind. Our results clearly demonstrated that copepods and krill had flexible feeding strategies to cope with temporally and spatially changing food availability in Prydz Bay. Meanwhile, ciliates appeared to represent an important carbon source for zooplankton, especially in regions with lower food concentrations.